Lens system and optical apparatus

ABSTRACT

A lens system comprising, in order from an object side: a first lens group G 1;  and a second lens group G 2  having positive refractive power; the first lens group G 1  including a sub-lens group GS 11  having positive refractive power, and a sub-lens group GS 12  having negative refractive power, the sub-lens group GS 12  having negative refractive power including a meniscus lens having a convex surface facing the object side, given conditions being satisfied, thereby providing a lens system having high optical performance with excellently correcting various aberrations, and an optical apparatus equipped therewith.

TECHNICAL FIELD

The present invention relates to a lens system suitable for aninterchangeable lens for a single-lens reflex camera and a photocopylens, and an optical apparatus equipped with the lens system.

BACKGROUND ART

A so-called double-Gauss type lens system has been used as a lens systemused for an interchangeable les for a single-lens reflex camera and aphotocopy lens, and a lot of lens systems have been proposed in such asJapanese Patent Application Laid-Open No. 2007-333790.

However, a conventional double-Gauss type lens system generates largecoma, so that it does not have sufficiently high optical performance.

DISCLOSURE OF THE INVENTION

The present invention is made in view of the above-described problem,and has an object to provide a lens system capable of accomplishingexcellent optical performance.

According to a first aspect of the present invention, there is provideda lens system comprising, in order from an object side: a first lensgroup; and a second lens group having positive refractive power; thefirst lens group including a sub-lens group having positive refractivepower, and a sub-lens group having negative refractive power, thesub-lens group having negative refractive power including a meniscuslens having a convex surface facing the object side, the followingconditional expressions being satisfied:

1.910<ndh

−0.400<f/f1<0.500

where ndh denotes a refractive index at d-line (wavelength=587.6 nm) ofa lens having the highest refractive index at d-line composing the lenssystem, f1 denotes a focal length of the first lens group, and f denotesa focal length of the lens system.

According to a second aspect of the present invention, there is providedan optical apparatus equipped with the lens system according to thefirst aspect.

According to a third aspect of the present invention, there is provideda lens system comprising, in order from an object side: a first lensgroup; and a second lens group having positive refractive power; thefirst lens group including a sub-lens group having positive refractivepower, and a sub-lens group having negative refractive power, thesub-lens group having negative refractive power including a meniscuslens having a convex surface facing the object side, the lens systemincluding at least one negative lens satisfying the followingconditional expressions:

1.820<nNh

−0.400<f/f1<0.500

where nNh denotes a refractive index at d-line (wavelength=587.6 nm) ofthe negative lens, f1 denotes a focal length of the first lens group,and f denotes a focal length of the lens system.

According to a fourth aspect of the present invention, there is providedan optical apparatus equipped with the lens system according to thethird aspect.

According to a fifth aspect of the present invention, there is provideda lens system comprising, in order from an object side: a first lensgroup; and a second lens group having positive refractive power; thefirst lens group including a sub-lens group having positive refractivepower, and a sub-lens group having negative refractive power, thesub-lens group having negative refractive power including a meniscuslens having a convex surface facing the object side, the followingconditional expressions being satisfied:

1.890<n2dh

−0.400<f/f1<0.500

where n2dh denotes a refractive index at d-line (wavelength=587.6 nm) ofa lens having the highest refractive index at d-line composing thesecond lens group, f1 denotes a focal length of the first lens group,and f denotes a focal length of the lens system.

According to a sixth aspect of the present invention, there is provideda method for manufacturing a lens system including, in order from anobject side along an optical axis, a first lens group and a second lensgroup including steps of: disposing a meniscus lens having a convexsurface facing the object side in a sub-lens group having negativerefractive power; disposing the first lens group and the second lensgroup in such a manner that the first lens group consists of a sub-lensgroup having positive refractive power and the sub-lens group havingnegative refractive power, and the second lens group has positiverefractive power with satisfying the following conditional expressions:

1.910<ndh

−0.400<f/f1<0.500

where ndh denotes a refractive index at d-line (wavelength=587.6 nm) ofa lens having the highest refractive index at d-line composing the lenssystem, f1 denotes a focal length of the first lens group, and f denotesa focal length of the lens system.

According to a seventh aspect of the present invention, there isprovided a method for manufacturing a lens system including, in orderfrom an object side along an optical axis, a first lens group and asecond lens group including steps of: disposing a meniscus lens having aconvex surface facing the object side in a sub-lens group havingnegative refractive power; disposing the first lens group and the secondlens group in such a manner that the first lens group consists of asub-lens group having positive refractive power and the sub-lens grouphaving negative refractive power, the second lens group has positiverefractive power, and the following conditional expressions aresatisfied:

1.820<nNh

−0.400<f/f1<0.500

where nNh denotes a refractive index at d-line (wavelength=587.6 nm) ofa negative lens composing the lens system, f1 denotes a focal length ofthe first lens group, and f denotes a focal length of the lens system.

The present invention makes it possible to provide a lens system havinghigh optical performance with excellently correcting variousaberrations, and an optical apparatus equipped with the lens system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a lens configuration of a lens systemaccording to Example 1.

FIGS. 2A and 2B are graphs showing various aberrations of the lenssystem according to Example 1, in which FIG. 2A shows upon focusing oninfinity (β=0.0), and FIG. 2B shows upon focusing on a close object (β=−1/30).

FIG. 3 is a sectional view showing a lens configuration of a lens systemaccording to Example 2.

FIGS. 4A and 4B are graphs showing various aberrations of the lenssystem according to Example 2, in which FIG. 4A shows upon focusing oninfinity (β=0.0), and FIG. 4B shows upon focusing on a close object (β=−1/30).

FIG. 5 is a sectional view showing a lens configuration of a lens systemaccording to Example 3.

FIGS. 6A and 6B are graphs showing various aberrations of the lenssystem according to Example 3, in which FIG. 6A shows upon focusing oninfinity (β=0.0), and FIG. 6B shows upon focusing on a close object (β=−1/30).

FIG. 7 is a sectional view showing a lens configuration of a lens systemaccording to Example 4.

FIGS. 8A and 8B are graphs showing various aberrations of the lenssystem according to Example 4, in which FIG. 8A shows upon focusing oninfinity (β=0.0), and FIG. 8B shows upon focusing on a close object (β=−1/30).

FIG. 9 is a sectional view showing a lens configuration of a lens systemaccording to Example 5.

FIGS. 10A and 10B are graphs showing various aberrations of the lenssystem according to Example 5, in which FIG. 10A shows upon focusing oninfinity (β=0.0), and FIG. 10B shows upon focusing on a close object(β=− 1/30).

FIG. 11 is a sectional view showing a lens configuration of a lenssystem according to Example 6.

FIGS. 12A and 12B are graphs showing various aberrations of the lenssystem according to Example 6, in which FIG. 12A shows upon focusing oninfinity (β=0.0), and FIG. 12B shows upon focusing on a close object(β=− 1/30).

FIG. 13 is a sectional view showing a lens configuration of a lenssystem according to Example 7.

FIGS. 14A and 14B are graphs showing various aberrations of the lenssystem according to Example 7, in which FIG. 14A shows upon focusing oninfinity (β=0.0), and FIG. 14B shows upon focusing on a close object(β=− 1/30).

FIG. 15 is a sectional view showing a lens configuration of a lenssystem according to Example 8.

FIGS. 16A and 16B are graphs showing various aberrations of the lenssystem according to Example 8, in which FIG. 16A shows upon focusing oninfinity (β=0.0), and FIG. 16B shows upon focusing on a close object(β=− 1/30).

FIG. 17 is a sectional view showing a lens configuration of a lenssystem according to Example 9.

FIGS. 18A and 18B are graphs showing various aberrations of the lenssystem according to Example 9, in which FIG. 18A shows upon focusing oninfinity (β=0.0), and FIG. 18B shows upon focusing on a close object(β=− 1/30).

FIG. 19 is a diagram showing a construction of a camera equipped withthe lens system according to Example 1.

FIG. 20 is a flowchart schematically explaining a method formanufacturing the lens system according to the present embodiment.

FIG. 21 is a flowchart schematically explaining a method formanufacturing the lens system according to the present embodiment seenfrom another point of view.

EMBODIMENT FOR CARRYING OUT THE INVENTION

A lens system according to an embodiment of the present application isexplained below.

A lens system according to the present embodiment includes, in orderfrom an object side along an optical axis, a first lens group, and asecond lens group having positive refractive power. In the lens system,the first lens group is composed of, in order from the object side, asub-lens group having positive refractive power, and a sub-lens grouphaving negative refractive power. The sub-lens group having negativerefractive power includes a meniscus lens having a convex surface facingthe object side. With this lens configuration, it becomes possible torealize so-called double-Gauss type refractive power distribution and toexcellently correct distortion and lateral chromatic aberration as wellas spherical aberration and curvature of field.

In a lens system according to the present embodiment, the followingconditional expressions (1) and (2) are satisfied:

1.910<ndh   (1)

−0.40<f/f1<0.5   (2)

where ndh denotes a refractive index at d-line (wavelength=587.6 nm) ofa lens having highest refractive index at d-line composing the lenssystem, and f1 denotes a focal length of the first lens group, and fdenotes a focal length of the lens system.

Conditional expression (1) is a condition for excellently correctingspherical aberration and sagittal coma generated in the lens system toobtain high optical performance.

When the value ndh falls below the lower limit of conditional expression(1), there are two cases. One is a case that the lens having highestrefractive power at d-line is a positive lens, and the other is a casethat the lens is a negative lens.

In the former case, since negative spherical aberration generated in thelens system becomes excessively large, in order to correct theaberration curvature of the negative lens is made large, in other words,the radius of curvature is made small. Accordingly, sagittal coma isgenerated excessively by the negative lens. In the latter case, sagittalcoma is excessively generated by the negative lens. In either case,higher optical performance cannot be obtained.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (1) to1.940. In order to secure the effect of the present embodiment, it ispreferable to set the value lower than 2.800. With setting the value ndhlower than 2.800, the lens system can be constructed with sufficientlysecuring transmittance in visible light of the optical material havingthe highest refractive power in the lens system.

Conditional expression (2) is for obtaining high optical performancewith excellently correcting distortion and lateral chromatic aberrationgenerated in the lens system.

When the ratio f/f1 falls below the lower limit of conditionalexpression (2), refractive power of the first lens group becomesexcessively large in negative direction. Accordingly, negativedistortion and lateral chromatic aberration become difficult to becorrected, so that high optical performance cannot be obtained.

On the other hand, when the ratio f/f1 exceeds the upper limit ofconditional expression (2), it becomes difficult to correct positivedistortion generated in the first lens group, and high opticalperformance cannot be obtained.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (2) to−0.250. In order to further secure the effect of the present embodiment,it is most preferable to set the lower limit of conditional expression(2) to −0.100. In order to further secure the effect of the presentembodiment, it is most preferable to set the lower limit of conditionalexpression (2) to 0.000. In order to still further secure the effect ofthe present embodiment, it is most preferable to set the lower limit ofconditional expression (2) to 0.100. In order to secure the effect ofthe present embodiment, it is preferable to set the upper limit ofconditional expression (2) to 0.400. In order to further secure theeffect of the present embodiment, it is most preferable to set the upperlimit of conditional expression (2) to 0.360.

In a lens system according to the present embodiment, the first lensgroup is preferably composed of, in order from the object side along theoptical axis, a sub-lens group having positive refractive power, and asub-lens group having negative refractive power.

With this configuration, it becomes possible to excellently correctdistortion and lateral chromatic aberration.

In a lens system according to the present embodiment, an aperture stopis preferably disposed between the first lens group and the second lensgroup.

With this configuration, it becomes possible to excellently correctdistortion and lateral chromatic aberration.

In a lens system according to the present embodiment, the sub-lens grouphaving positive refractive power includes a lens element having positiverefractive power disposed to the most object side, and in the lenselement, an absolute value of a radius of curvature of an object sidesurface is preferably smaller than an absolute value of a radius ofcurvature of an image side surface.

With this configuration, it becomes possible to gradually bend light rayproceeding toward the center of the image by the lens element. As aresult, aberrations generated in the lens element, in particular,generation of spherical aberration can be suppressed, so that highoptical performance can be obtained.

Incidentally, a lens element in the present application is a genericname of a single lens and a cemented lens.

In a lens system according to the present embodiment, the sub-lens grouphaving positive refractive power is preferably composed of only apositive lens element.

With this configuration, the number of lenses composing the lens systemcan be reduced, flare caused by reflections from lens surfaces can bemitigated, and high optical performance can be realized.

In a lens system according to the present embodiment, the followingconditional expression (3) is preferably satisfied:

24.0<νdh   (3)

where νdh denotes an Abbe number at d-line of a lens having the highestrefractive index at d-line.

Conditional expression (3) is for obtaining high optical performancewith suppressing chromatic aberration.

When the value νdh falls below the lower limit of conditional expression(3), and when the lens having highest refractive index at d-line is apositive lens, correction of chromatic aberration becomes insufficient.When the lens having highest refractive index at d-line is a negativelens, correction of chromatic aberration becomes excessive, in eithercases high optical performance cannot be obtained.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (3) to 25.0.In order to further secure the effect of the present embodiment, it ispreferable to set the upper limit of conditional expression (3) to 30.0.With setting νdh smaller than 30.0, it becomes possible to excellentlycorrect chromatic difference in spherical aberration, so that highoptical performance can be obtained.

In a lens system according to the present embodiment, the lens systempreferably includes at least one lens having negative refractive powerthat satisfies the following conditional expression (4):

1.820<nNh   (4)

where nNh denotes a refractive index at d-line (wavelength=587.6 nm) ofthe negative lens.

Conditional expression (4) is for obtaining high optical performancewith excellently correcting spherical aberration and sagittal comagenerated in the lens system.

When the value nNh falls below the lower limit of conditional expression(4), sagittal coma is excessively generated in the negative lens, sothat, high optical performance cannot be obtained. In order to securethe effect of the present embodiment, it is preferable to set the lowerlimit of conditional expression (4) to 1.840. In order to further securethe effect of the present embodiment, it is most preferable to set thelower limit of conditional expression (4) to 1.860. In order to securethe effect of the present embodiment, it is preferable to set the upperlimit of conditional expression (4) to 2.80. With setting nNh smallerthan 2.800, it becomes possible to sufficiently secure transmittance invisible light range of the optical material of the negative lens.

In a lens system according to the present embodiment, the most imageside lens of the sub-lens group having negative refractive power ispreferably a lens with negative refractive power having a concavesurface facing the image side.

In a lens system according to the present embodiment, the first lensgroup is composed of the sub-lens group having positive refractive powerand the sub-lens group having negative refractive power. Negativespherical aberration generated in the sub-lens group having positiverefractive power comes to be excellently corrected by composing the mostimage side in the sub-lens group having negative refractive power withthe lens with negative refractive power having a concave surface facingthe image side, so that generation of spherical aberration can besuppressed in low level by the whole of the first lens group. Moreover,coma can be corrected at the same time, so that high optical performancecan be obtained as a whole of the lens system.

In a lens system according to the present embodiment, the second lensgroup preferably includes at least one negative lens having a concavesurface facing the object side, and the following conditional expression(5) is preferably satisfied:

0.300<|r2Na|/f<0.600   (5)

where r2Na denotes a radius of curvature of the object side surface ofthe at least one negative lens.

Conditional expression (5) is for realizing high optical performancewith suppressing sagittal coma.

When the ratio |r2Na|/f falls below the lower limit of conditionalexpression (5), it becomes difficult to excellently correct negativespherical aberration generated in the second lens group. Moreover, uponfocusing from infinity to a close range by the lens system, or uponchanging magnification by a projection apparatus or a photocopy machineequipped with a lens system according to the present embodiment,variation in aberrations becomes excessively large, so that high opticalperformance cannot be secured from infinity to a close range or overbroad magnification range.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (5) to0.320. In order to secure the effect of the present embodiment, it ispreferable to set the upper limit of conditional expression (5) to0.530.

In a lens system according to the present embodiment, the at least onenegative lens having a concave surface facing the object side ispreferable disposed to the most object side in the second lens group.

With disposing at least on negative lens having a concave surface facingthe object side to the most object side in the second lens group, itbecomes possible to obtain high optical performance with suppressingdistortion and lateral chromatic aberration generated in the negativelens

In a lens system according to the present embodiment, the followingconditional expression (6) is preferably satisfied:

0.800<|r2a|/r1b<1.200   (6)

where r1b denotes a radius of curvature of the most image side lenssurface of the first lens group, and r2a denotes a radius of curvatureof the most object side lens surface of the second lens group.

Conditional expression (6) is for realizing high optical performancewith suppressing sagittal coma.

When the ratio |r2a|/r1b falls below the lower limit of conditionalexpression (6), in other words, when the curvature of the most objectside lens surface of the second lens group becomes excessively largewith respect to the curvature of the most image side lens surface of thefirst lens group, sagittal coma is excessively generated on the objectside surface of the second lens group, so that high optical performancecannot be realized.

When the ratio |r2a|/r1b exceeds the upper limit of conditionalexpression (6), in other words, when the curvature of the most imageside lens surface of the first lens group becomes excessively large withrespect to the curvature of the most object side lens surface of thesecond lens group, sagittal coma is excessively generated on the imageside surface of the first lens group, so that high optical performancecannot be realized.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (6) to0.900. In order to secure the effect of the present embodiment, it ispreferable to set the upper limit of conditional expression (6) to1.150. In order to further secure the effect of the present embodiment,it is most preferable to set the upper limit of conditional expression(6) to 1.100.

In a lens system according to the present embodiment, the followingconditional expression (7) is preferably satisfied:

0.600<Bf/f<1.00   (7)

where Bf denotes a distance along an optical axis between the most imageside lens surface of the lens system and the image plane.

Conditional expression (7) is for realizing high optical performance.

When the ratio Bf/f falls below the lower limit of conditionalexpression (7), the back focal length becomes relatively short withrespect to the focal length of the lens system, power distribution ofthe lens system becomes largely away from a symmetrical type, and itbecomes difficult to correct distortion, so that high opticalperformance cannot be realized.

On the other hand, when the ratio Bf/f exceeds the upper limit ofconditional expression (7), the back focal length becomes relativelylong with respect to the focal length of the lens system, powerdistribution of the lens system becomes largely away from a symmetricaltype, and it becomes difficult to correct distortion, so that highoptical performance cannot be realized.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (7) to0.650. In order to further secure the effect of the present embodiment,it is most preferable to set the lower limit of conditional expression(7) to 0.700. In order to secure the effect of the present embodiment,it is preferable to set the upper limit of conditional expression (7) to0.850. In order to further secure the effect of the present embodiment,it is most preferable to set the upper limit of conditional expression(7) to 0.800.

Incidentally, when a plane parallel plate is inserted between the mostimage side lens surface and the image plane, Bf is an equivalent airthickness.

In a lens system according to the present embodiment, a distance betweenthe first lens group and the second lens group is preferably alwaysfixed.

With this configuration, it becomes possible to move the first lensgroup and the second lens group in a body upon focusing from infinity toa close range or upon changing magnification by a projection apparatusor a photocopy machine equipped with a lens system according to thepresent embodiment, so that generation of decentering coma generated bydecentering the first lens group and the second lens group can besuppressed in comparison with a case that the first lens group and thesecond lens group are moves separately. Accordingly, high opticalperformance can be realized from infinity to a close range or over broadrange of a variable magnification range

Then, a lens system according to the present embodiment seen fromanother point of view includes, in order from an object side along anoptical axis, a first lens group, and a second lens group havingpositive refractive power. The first lens group is composed of asub-lens group having positive refractive power, and a sub-lens grouphaving negative refractive power. The sub-lens group having negativerefractive power includes a meniscus lens having a convex surface facingthe object side. With this lens configuration, it becomes possible torealize a so-called double-Gauss type refractive power distribution andto excellently correct distortion and lateral chromatic aberration aswell as spherical aberration and curvature of field.

Moreover, in a lens system according to the present embodiment seen fromanother point of view, the lens system includes at least one negativelens that satisfies the following conditional expression (4), and thefollowing conditional expressions (4) and (2) are satisfied:

1.820<nNh   (4)

−0.400<f/f1<0.500   (2)

where nNh denotes a refractive index at d-line (wavelength=587.6 nm) ofthe negative lens, and f1 denotes a focal length of the first lensgroup, and f denotes a focal length of the lens system.

Conditional expression (4) is for obtaining high optical performancewith excellently correcting spherical aberration and sagittal comagenerated in the lens system. However, conditional expression (4) hasalready explained above, so that duplicated explanations are omitted.

Conditional expression (2) is for obtaining high optical performancewith excellently correcting distortion and lateral chromatic aberrationgenerated in the lens system. However, conditional expression (2) hasalready explained above, so that duplicated explanations are omitted.

In a lens system according to the present embodiment seen from anotherpoint of view, the following conditional expression (8) is preferablysatisfied:

12.0<νdN<24.0   (8)

where νdN denotes an Abbe number at d-line of the negative lens.

Conditional expression (8) is for obtaining high optical performancewith suppressing chromatic aberration.

When the value νdN falls below the lower limit of conditional expression(8), chromatic variation in spherical aberration is correctedexcessively, so that high optical performance cannot be obtained. On theother hand, when the value νdN exceeds the upper limit of conditionalexpression (8), chromatic variation in spherical aberration is correctedinsufficiently, so that high optical performance cannot be obtained.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (8) to 16.0.In order to further secure the effect of the present embodiment, it ismost preferable to set the lower limit of conditional expression (8) to18.0.

In a lens system according to the present embodiment seen from anotherpoint of view, the negative lens preferably has a meniscus shape.

With this configuration, it becomes possible to obtain excellent opticalperformance with suppressing generation of coma.

In a lens system according to the present embodiment seen from anotherpoint of view, the negative lens is preferably disposed to the mostobject side in the second lens group, and the object side surface of thenegative lens is preferably a concave surface facing the object side.

With this configuration, it becomes possible to suppress generation ofcoma, in particular, sagittal coma to obtain high optical performance.

In a lens system according to the present embodiment seen from anotherpoint of view, the negative lens is preferably disposed to the mostimage side in the first lens group, and the image side surface of thenegative lens is preferably a concave surface facing the image side.

With this configuration, it becomes possible to suppress generation ofcoma, in particular, sagittal coma to obtain high optical performance.

Then, a lens system according to the present embodiment seen from stillanother point of view is explained.

A lens system according to the present embodiment seen from stillanother point of view includes, in order from an object along an opticalaxis, a first lens group and a second lens group having positiverefractive power. The first lens group is composed of a sub-lens grouphaving positive refractive power, and a sub-lens group having negativerefractive power. The sub-lens group having negative refractive powerincludes a lens having a meniscus shape with a convex surface facing theobject side. With this lens configuration, it becomes possible torealize a so-called double-Gauss type refractive power distribution andto excellently correct distortion and lateral chromatic aberration aswell as spherical aberration and curvature of field.

In a lens system according to the present embodiment seen from stillanother point of view, the following conditional expressions (9) and (2)are satisfied:

1.890<n2dh   (9)

−0.400<f/f1<0.500   (2)

where n2dh denotes a refractive index at d-line (wavelength=587.6 nm) ofa lens having the highest refractive index at d-line composing thesecond lens group, f1 denotes a focal length of the first lens group,and f denotes a focal length of the lens system.

Conditional expression (9) is for obtaining high optical performancewith excellently correcting spherical aberration and sagittal comagenerated in the lens system.

When the value n2dh falls below the lower limit of conditionalexpression (9), there are two cases that the lens having highestrefractive index at d-line is a positive lens, and the lens is anegative lens.

In the former case, negative spherical aberration generated in the lenssystem becomes excessively large. In order to correct it, refractivepower of the negative lens in the lens system is made large, in otherwords, the radius of curvature is made small, so that sagittal coma isexcessively generated by the negative lens. In the latter case, sagittalcoma is excessively generated by the negative lens. Accordingly, ineither case, high optical performance cannot be obtained.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (9) to1.900. In order to further secure the effect of the present embodiment,it is most preferable to set the lower limit of conditional expression(9) to 1.910. In order to still further secure the effect of the presentembodiment, it is most preferable to set the lower limit of conditionalexpression (9) to 1.940. In order to secure the effect of the presentembodiment, it is preferable to set the value n2dh smaller than 2.800.With setting the value n2dh smaller than 2.800, the lens system can beconstructed with sufficiently securing transmittance in visible light ofthe optical material having the highest refractive power at d-line inthe second lens group.

Conditional expression (2) is for obtaining high optical performancewith excellently correcting distortion and lateral chromatic aberrationgenerated in the lens system. However, conditional expression (2) hasalready explained above, so that duplicated explanations are omitted.

In a lens system according to the present embodiment seen from stillanother point of view, the following conditional expression (6A) ispreferably satisfied:

0.750<|r2a|/r1b<1.250   (6A)

where r1b denotes a radius of curvature of the most image side lenssurface in the first lens group, and r2a denotes a radius of curvatureof the most object side lens surface in the second lens group.

Conditional expression (6A) is for realizing high optical performancewith suppressing sagittal coma.

When the ratio |r2a|/r1b falls below the lower limit of conditionalexpression (6A), in other words, when the curvature of the most objectside lens surface of the second lens group becomes excessively largewith respect to the curvature of the most image side lens surface of thefirst lens group, sagittal coma is excessively generated on the objectside surface of the second lens group, so that high optical performancecannot be realized.

When the ratio |r2a|/r1b exceeds the upper limit of conditionalexpression (6A), in other words, when the curvature of the most imageside lens surface of the first lens group becomes excessively large withrespect to the curvature of the most object side lens surface of thesecond lens group, sagittal coma is excessively generated on the imageside surface of the first lens group, so that high optical performancecannot be realized.

In order to secure the effect of the present embodiment, it ispreferable to set the lower limit of conditional expression (6A) to0.850. In order to secure the effect of the present embodiment, it ispreferable to set the upper limit of conditional expression (6A) to1.220. In order to further secure the effect of the present embodiment,it is most preferable to set the upper limit of conditional expression(6A) to 1.150.

Moreover, any lens surface may be formed as a spherical surface, a planesurface or an aspherical surface.

When a lens surface is a spherical surface or a plane surface,processing and assembly of the lens become easy, so that deteriorationof optical performance caused by errors upon processing and assemblingcan be prevented. Even if the lens surface is shifted, deterioration inoptical performance is small, so that it is desirable.

When the lens surface is an aspherical surface, the aspherical surfacemay be fabricated by a fine grinding process, a glass molding processthat a glass material is formed into an aspherical shape by a mold, or acompound type process that a resin material is formed into an asphericalshape on a glass surface. Any lens surface may be a diffractive opticalsurface. Any lens may be a graded index lens (GRIN lens), or a plasticlens.

Each example according to the present embodiment is explained below withreference to accompanying drawings. Incidentally, a detailed explanationof antireflection coating is separately shown after Examples.

EXAMPLE 1

FIG. 1 is a sectional view showing a lens configuration of a lens systemaccording to Example 1.

The lens system according to Example 1 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a positivemeniscus lens L121 having a convex surface facing the object side withstronger refractive power than the image side surface and a negativemeniscus lens L12R having a concave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a positive meniscus lens L23 having a convex surface facing theimage side, and a double convex positive lens L24 disposed to the mostimage side with stronger refractive power on the object side surfacewith respect to the image side surface. Light ray come out from the lensL24 forms an image on the image plane T.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 1are listed in Table 1.

In [Specifications], f denotes a focal length of a lens system, FNOdenotes an f-number, ω denotes a half angle of view (unit: degree), Ydenotes an image height, and TL denotes a total lens length that is adistance between the object side surface of the lens L111 upon focusinginfinity and an image plane I.

In [Lens Data], “i” shows optical surface number, “r” shows a radius ofcurvature of each optical surface (mm), “d” shows a surface distance(mm), “nd” shows a refractive index at d-line (wavelength: 587.56 nm),and “νd” shows an Abbe number at d-line. Incidentally, refractive indexof the air nd=1.000000 is omitted, and “r=∞” indicates a plane surface.

In [Variable Distances], INF denotes a state upon focusing on infinity,CLD denotes a state upon focusing on a close object, R denotes ashooting range that is a distance between an object and an image plane I(unit: m), β denotes a shooting magnification, and Bf denotes a backfocal length.

In [Values for Conditional Expressions], values for conditionalexpressions are shown.

In respective tables for various values, “mm” is generally used for theunit of length such as the focal length, the radius of curvature and thedistance to the next lens surface. However, since similar opticalperformance can be obtained by an optical system proportionally enlargedor reduced its dimension, the unit is not necessarily to be limited to“mm”, and any other suitable unit can be used. The explanation ofreference symbols is the same in the other Examples.

TABLE 1 [Specifications] f = 51.61 FNO = 1.45 ω = 23.10 Y = 21.60 TL =85.66 [Lens Data] i r d nd νd 1 61.1028 5.2000 1.834807 42.71 2 421.60370.1000 3 26.6848 4.5000 2.003300 28.27 4 42.4965 1.8000 5 55.6668 2.00001.808090 22.79 6 18.6474 9.3000 7 ∞ 7.7000 Aperture stop S 8 −18.71111.8000 1.846660 23.78 9 −64.8673 6.0000 1.788001 47.37 10  −29.03810.2000 11  −90.5334 5.5000 1.834807 42.71 12  −33.0755 0.1000 13 114.2530 3.0000 1.772499 49.60 14  −161.9377 (Bf) [Variable Distances]INF CLD R = ∞ 1.64 β = 0.0 −1/30 Bf = 38.4641 40.1846 [Values forConditional Expressions] (1) ndh = 2.003300 (2) f/f1 = 0.33461 (3) νdh =28.27 (4) nNh = 1.846660 (5) |r2Na|/f = 0.36253 (6) |r2a|/r1b = 1.00342(7) Bf/f = 0.74525 (8) νdN = 23.78

In the lens system according to Example 1, the lens having highestrefractive power at d-line is the lens L121. The lens L21 is thenegative lens LN satisfying conditional expression (4), and the negativelens L2N having a concave surface facing the object side. Here, theradius of curvature of the object side surface of the lens L21 is r2Naand r2 a, and the radius of curvature of the image side surface of thelens L12R is rib.

FIGS. 2A and 2B are graphs showing various aberrations of the lenssystem according to Example 1, in which FIG. 2A shows upon focusing oninfinity (β=0.0), and FIG. 2B shows upon focusing on a close object (β=−1/30).

In respective graphs, FNO denotes an f-number, A denotes a half angle ofview, NA denotes a numerical aperture, H0 denotes an object height(unit: mm). Moreover, d denotes an aberration curve at d-line(wavelength λ=587.6 nm), g denotes an aberration curve at g-line(wavelength λ=435.6 nm), and an aberration curve without mentioningdenotes various aberrations with respect to d-line. In graphs showingastigmatism, a solid line indicates a sagittal image plane, and a brokenline indicates a meridional image plane. In graphs showing coma, in eachhalf angle of view or each object height, solid lines denote meridionalcoma at d-line and g-line, a broken line shown in the left side of theorigin shows sagittal coma generated in meridional direction at d-line,and a broken line shown in right side of the origin shows sagittal comagenerated in sagittal direction at d-line.

Incidentally, the same notation is used in the following Examples, sothat duplicated explanations are omitted.

As is apparent from the respective graphs, the lens system according toExample 1 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 2

FIG. 3 is a sectional view showing a lens configuration of a lens systemaccording to Example 2.

The lens system according to Example 2 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 havingnegative refractive power is constructed by, in order from the objectside along the optical axis, a positive meniscus lens L121 having aconvex surface facing the object side with stronger refractive powerthan the image side surface and a negative meniscus lens L12R having aconcave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a positive meniscus lens L23 having a convex surface facing theimage side, and a double convex positive lens L24 disposed to the mostimage side with stronger refractive power on the object side surfacewith respect to the image side surface. Light ray come out from the lensL24 forms an image on the image plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 2are listed in Table 2.

TABLE 2 [Specifications] f = 51.61 FNO = 1.45 ω = 23.07 Y = 21.60 TL =86.39 [Lens Data] i r d nd νd 1 52.2387 6.0000 1.834807 42.71 2 565.27510.1000 3 28.1185 4.4000 2.000690 25.46 4 45.6886 1.5000 5 78.8809 2.00001.860740 23.06 6 19.8082 9.3000 7 ∞ 7.7000 Aperture Stop S 8 −19.20241.8000 1.805181 25.42 9 −125.5269 6.0000 1.788001 47.37 10  −29.93310.2000 11  −92.9638 5.5000 1.834807 42.71 12  −33.8967 0.1000 13 112.4183 3.3000 1.772499 49.60 14  −161.9377 (Bf) [Variable Distances]INF CLD R = ∞ 1.64 β = 0.0 −1/30 Bf = 38.4874 40.2076 [Values forConditional Expressions] (1) ndh = 2.000690 (2) f/f1 = 0.29794 (3) νdh =25.46 (4) nNh = 1.860740 (5) |r2Na|/f = 0.37209 (6) |r2a|/r1b = 0.96941(7) Bf/f = 0.74579 (8) νdN = 23.06

In the lens system according to Example 2, the lens having highestrefractive power at d-line is the lens L121. The lens L12R is thenegative lens LN satisfying conditional expression (4), and the lens L21is the negative lens L2N having a concave surface facing the objectside. Here, the radius of curvature of the object side surface of thelens L21 is r2Na and r2a, and the radius of curvature of the image sidesurface of the lens L12R is r1b.

FIGS. 4A and 4B are graphs showing various aberrations of the lenssystem according to Example 2, in which FIG. 4A shows upon focusing oninfinity (β=0.0), and FIG. 4B shows upon focusing on a close object (β=−1/30).

As is apparent from the respective graphs, the lens system according toExample 2 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 3

FIG. 5 is a sectional view showing a lens configuration of a lens systemaccording to Example 3.

The lens system according to Example 3 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a positivemeniscus lens L121 having a convex surface facing the object side withstronger refractive power than the image side surface and a negativemeniscus lens L12R having a concave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a positive meniscus lens L23 having a convex surface facing theimage side, and a double convex positive lens L24 disposed to the mostimage side with stronger refractive power on the object side surfacewith respect to the image side surface. Light ray come out from the lensL24 forms an image on the image plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 3are listed in Table 3.

TABLE 3 [Specifications] f = 51.61 FNO = 1.45 ω = 23.14 Y = 21.60 TL =85.48 [Lens Data] i r d nd νd 1 61.1431 4.8000 1.882997 40.76 2 389.57560.3000 3 26.8932 4.5000 2.000690 25.46 4 44.9000 2.2000 5 57.4287 2.00001.922860 20.50 6 19.2263 8.0000 7 ∞ 7.8000 Aperture Stop S 8 −18.65581.8000 1.805181 25.42 9 −114.6796 6.0000 1.788001 47.37 10  −30.11340.3000 11  −114.9741 6.0000 1.834807 42.71 12  −34.5098 0.1000 13 126.4714 3.2000 1.804000 46.57 14  −161.9377 (Bf) [Variable Distances]INF CLD R = ∞ 1.64 β = 0.0 −1/30 Bf = 38.4783 40.1986 [Values forConditional Expressions] (1) ndh = 2.000690 (2) f/f1 = 0.30465 (3) νdh =25.46 (4) nNh = 1.922860 (5) |r2Na|/f = 0.36149 (6) |r2a|/r1b = 0.97033(7) Bf/f = 0.74558 (8) νdN = 20.50

In the lens system according to Example 3, the lens having highestrefractive power at d-line is the lens L121. The lens L12R is thenegative lens LN satisfying conditional expression (4), and the lens L21is the negative lens L2N having a concave surface facing the objectside. Here, the radius of curvature of the object side surface of thelens L21 is r2Na and r2a, and the radius of curvature of the image sidesurface of the lens L12R is r1b.

FIGS. 6A and 6B are graphs showing various aberrations of the lenssystem according to Example 3, in which FIG. 6A shows upon focusing oninfinity (β=0.0), and FIG. 6B shows upon focusing on a close object (β=−1/30).

As is apparent from the respective graphs, the lens system according toExample 3 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 4

FIG. 7 is a sectional view showing a lens configuration of a lens systemaccording to Example 4.

The lens system according to Example 4 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a positivemeniscus lens L121 having a convex surface facing the object side withstronger refractive power than the image side surface and a negativemeniscus lens L12R having a concave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a double concavenegative lens L21 having stronger refractive power on the object sidesurface than on the image side surface cemented with a double convexpositive lens L22 having weaker refractive power on the object sidesurface than on the image side surface, a positive meniscus lens L23having a convex surface facing the image side, a positive meniscus lensL24 having a convex surface facing the image side, and a double convexpositive lens L25 disposed to the most image side with strongerrefractive power on the object side surface with respect to the imageside surface. Light ray come out from the lens L25 forms an image on theimage plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 4are listed in Table 4.

TABLE 4 [Specifications] f = 51.63 FNO = 1.25 ω = 23.33 Y = 21.60 TL =94.94 [Lens Data] i r d nd νd 1 57.5740 5.5000 1.903660 31.31 2 206.09060.2000 3 29.5832 6.0000 2.003300 28.27 4 38.8499 2.0000 5 41.1771 2.00001.922860 18.90 6 19.7787 9.5000 7 ∞ 9.2000 Aperture Stop S 8 −19.41662.0000 1.805181 25.42 9 3374.4434 7.0000 1.804000 46.57 10  −36.67790.2000 11  −56.5118 4.0000 1.882997 40.76 12  −42.4595 0.1000 13 −143.9382 5.0000 1.834807 42.71 14  −44.3487 0.3000 15  103.1998 4.00001.754999 52.32 16  −161.9377 (Bf) [Variable Distances] INF CLD R = ∞1.64 β = 0.0 −1/30 Bf = 37.9374 39.6583 [Values for ConditionalExpressions] (1) ndh = 2.003300 (2) f/f1 = 0.29576 (3) νdh = 28.27 (4)nNh = 1.922860 (5) |r2Na|/f = 0.37608 (6) |r2a|/r1b = 0.98169 (7) Bf/f =0.73481 (8) νdN = 18.90

In the lens system according to Example 4, the lens having highestrefractive power at d-line is the lens L121. The lens L12R is thenegative lens LN satisfying conditional expression (4), and the lens L21is the negative lens L2N having a concave surface facing the objectside. Here, the radius of curvature of the object side surface of thelens L21 is r2Na and r2a, and the radius of curvature of the image sidesurface of the lens L12R is r1b.

FIGS. 8A and 8B are graphs showing various aberrations of the lenssystem according to Example 4, in which FIG. 8A shows upon focusing oninfinity (β=0.0), and FIG. 8B shows upon focusing on a close object (β=−1/30).

As is apparent from the respective graphs, the lens system according toExample 4 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 5

FIG. 9 is a sectional view showing a lens configuration of a lens systemaccording to Example 5.

The lens system according to Example 5 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a positivemeniscus lens L121 having a convex surface facing the object side withstronger refractive power than the image side surface and a negativemeniscus lens L12R having a concave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a positive meniscus lens L23 having a convex surface facing theimage side, and a cemented lens constructed by a positive lens L24having a convex surface facing the image side whose radius of curvatureis smaller than that of the object side surface cemented with a negativemeniscus lens L25 having a concave surface facing the object side. Lightray come out from the lens L25 forms an image on the image plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 5are listed in Table 5.

TABLE 5 [Specifications] f = 51.61 FNO = 1.45 ω = 23.13 Y = 21.60 TL =87.36 [Lens Data] i r d nd νd 1 62.5707 4.6000 1.834807 42.71 21079.5273 0.1000 3 27.8402 4.5000 1.903660 31.31 4 48.9187 2.0000 583.6288 1.6000 1.805181 25.42 6 20.6020 9.5000 7 ∞ 7.7000 Aperture StopS 8 −19.6349 1.8000 1.846660 23.78 9 −36.0084 6.0000 1.754999 52.32 10 −28.9194 0.2000 11  −100.0147 4.5000 1.834807 42.71 12  −35.2169 0.100013  140.8843 5.0000 2.003300 28.27 14  −49.8565 1.5000 1.922860 18.9015  −391.9566 (Bf) [Variable Distances] INF CLD R = ∞ 1.65 β = 0.0 −1/30Bf = 38.2645 39.9850 [Values for Conditional Expressions] (1) ndh =2.003300 (2) f/f1 = 0.28536 (3) νdh = 28.27 (4) nNh = 1.846660 (L21) (4)nNh = 1.922860 (L25) (5) |r2Na|/f = 0.38041 (L21) (6), (6A) |r2a|/r1b =0.95306 (7) Bf/f = 0.74135 (8) νdN = 23.78 (L21) (8) νdN = 18.90 (L25)(9) n2dh = 2.003300

In the lens system according to Example 5, the lens having highestrefractive power at d-line is the lens L24. The lenses L21 and L25 arethe negative lens LN satisfying conditional expression (4), and thenegative lens L2N having a concave surface facing the object side. Here,the radius of curvature of the object side surface of the lens L21 isr2Na and r2a, and the radius of curvature of the image side surface ofthe lens L12R is r1b.

FIGS. 10A and 10B are graphs showing various aberrations of the lenssystem according to Example 5, in which FIG. 10A shows upon focusing oninfinity (β=0.0), and FIG. 10B shows upon focusing on a close object(β=− 1/30).

As is apparent from the respective graphs, the lens system according toExample 5 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 6

FIG. 11 is a sectional view showing a lens configuration of a lenssystem according to Example 6.

The lens system according to Example 6 is composed of, in order from anobject side along an optical axis, a first lens group G1 having negativerefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a negativemeniscus lens L121 having a convex surface facing the object side and apositive meniscus lens L12R having a concave surface facing the imageside.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a positive meniscus lens L23 having a convex surface facing theimage side, and a double convex positive lens L24 disposed to the mostimage side with stronger refractive power on the image side surface withrespect to the object side surface. Light ray come out from the lens L24forms an image on the image plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

In the lens system according to Example 6, although the lens L111 isconstructed by a single lens, the lens may be constructed by a cementedlens. In this case, spherical aberration and longitudinal chromaticaberration can be excellently corrected, so that it is desirable.

Various values associated with the lens system according to Example 6are listed in Table 6.

TABLE 6 [Specifications] f = 51.60 FNO = 2.10 ω = 25.28 Y = 24.00 TL =108.47 [Lens Data] i r d nd νd 1 51.0799 4.0000 1.799516 42.22 2153.5021 1.0000 3 25.0934 2.0000 1.834807 42.71 4 16.6855 7.0000 517.7942 2.5000 2.019600 21.45 6 17.4360 8.0000 7 ∞ 9.0000 Aperture StopS 8 −20.1623 1.5000 1.860740 23.06 9 −141.0470 8.0000 1.754999 52.32 10 −24.1458 1.0000 11  −82.3047 4.5000 1.834807 42.71 12  −39.2143 0.500013  187.0181 3.5000 1.804000 46.57 14  −161.9377 (Bf) [VariableDistances] INF CLD R = ∞ 1.66 β = 0.0 −1/30 Bf = 55.9690 57.6889 [Valuesfor Conditional Expressions] (1) ndh = 2.019600 (2) f/f1 = −0.02715 (4)nNh = 1.834807 (L121) (4) nNh = 1.860740 (L21) (5) |r2Na|/f = 0.39073(6) |r2a|/r1b = 1.15636 (8) νdN = 23.06 (L21)

In the lens system according to Example 6, the lens having highestrefractive power at d-line is the lens L12R. The lenses L121 and L21 arethe negative lens LN satisfying conditional expression (4), and the lensL21 is also the negative lens L2N having a concave surface facing theobject side. Here, the radius of curvature of the object side surface ofthe lens L21 is r2Na and r2a, and the radius of curvature of the imageside surface of the lens L12R is r1b.

FIGS. 12A and 12B are graphs showing various aberrations of the lenssystem according to Example 6, in which FIG. 12A shows upon focusing oninfinity (β=0.0), and FIG. 12B shows upon focusing on a close object(β=− 1/30).

As is apparent from the respective graphs, the lens system according toExample 6 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 7

FIG. 13 is a sectional view showing a lens configuration of a lenssystem according to Example 7.

The lens system according to Example 7 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a positivemeniscus lens L121 having a convex surface facing the object side withstronger refractive power than the image side surface and a negativemeniscus lens L12R having a concave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a cemented lens constructed by a negative meniscus lens L23having a concave surface facing the object side cemented with a positivemeniscus lens L24 having a concave surface facing the object side withweaker refractive power than the image side surface, a positive meniscuslens L25 having a convex surface facing the image side, and a doubleconvex positive lens L26 disposed to the most image side with strongerrefractive power on the object side surface with respect to the imageside surface. Light ray come out from the lens L26 forms an image on theimage plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 7are listed in Table 7.

TABLE 7 [Specifications] f = 51.60 FNO = 1.25 ω = 23.29 Y = 21.60 TL =95.78 [Lens Data] i r d nd νd 1 81.4768 5.0000 1.804000 46.57 2−2838.8241 0.3000 3 33.1597 7.0000 1.950300 29.42 4 60.4403 1.4000 595.3280 2.0000 1.761821 26.52 6 23.0471 11.3000 7 ∞ 7.0000 Aperture StopS 8 −26.4313 1.6000 1.805181 25.42 9 −125.7273 4.6000 1.804000 46.57 10 −32.7268 2.7000 11  −23.4449 1.7000 1.805181 25.42 12  −64.8415 5.00001.882997 40.76 13  −31.7094 0.1000 14  −187.5490 4.2000 1.882997 40.7615  −47.3960 0.1000 16  79.1117 3.8000 1.804000 46.57 17  −1019.1299(Bf) [Variable Distances] INF CLD R = ∞ 1.64 β = 0.0 −1/30 Bf = 37.979639.6996 [Values for Conditional Expressions] (1) ndh = 1.950300 (2) f/f1= 0.30850 (3) νdh = 29.42 (5) |r2Na|/f = 0.51223 (L21) (5) |r2Na|/f =0.45435 (L23) (6) |r2a|/r1b = 1.14684 (7) Bf/f = 0.73603

In the lens system according to Example 7, the lens having highestrefractive power at d-line is the lens L121. The lenses L21 and L23 arethe negative lens L2N having a concave surface facing the object side.Here, the radius of curvature of the object side surface of the lens L21is r2Na and r2a, and the radius of curvature of the image side surfaceof the lens L12R is r1b. The radius of curvature of the object sidesurface of the lens L23 is r2Na.

FIGS. 14A and 14B are graphs showing various aberrations of the lenssystem according to Example 7, in which FIG. 14A shows upon focusing oninfinity (β=0.0), and FIG. 14B shows upon focusing on a close object (β−1/30).

As is apparent from the respective graphs, the lens system according toExample 7 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 8

FIG. 15 is a sectional view showing a lens configuration of a lenssystem according to Example 8.

The lens system according to Example 8 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a positivemeniscus lens L121 having a convex surface facing the object side withstronger refractive power than the image side surface and a negativemeniscus lens L12R having a concave surface facing the image side.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a negativemeniscus lens L21 having a concave surface facing the object sidecemented with a positive meniscus lens L22 having a concave surfacefacing the object side with weaker refractive power than the image sidesurface, a positive meniscus lens L23 having a convex surface facing theimage side, and a double convex positive lens L24 disposed to the mostimage side with stronger refractive power on the object side surfacewith respect to the image side surface. Light ray come out from the lensL24 forms an image on the image plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

Various values associated with the lens system according to Example 8are listed in Table 8.

TABLE 8 [Specifications] f = 51.62 FNO = 1.44 ω = 23.19 Y = 21.60 TL =86.13 [Lens Data] i r d nd νd 1 61.9292 5.2000 1.834807 42.71 2 250.92820.1000 3 25.5456 4.5000 2.003300 28.27 4 37.7633 1.8000 5 40.4696 2.00001.808090 22.79 6 17.5981 9.3000 7 ∞ 7.7000 Aperture Stop S 8 −17.80811.8000 1.846660 23.78 9 −205.7528 6.0000 1.788001 47.37 10  −27.62090.2000 11  −110.8983 5.5000 1.804000 46.57 12  −31.7495 0.1000 13 136.7328 3.0000 2.003300 28.27 14  −375.0666 (Bf) [Variable Distances]INF CLD R = ∞ 1.64 β = 0.0 −1/30 Bf = 38.9302 40.6510 [Values forConditional Expressions] (2) f/f1 = 0.33611 (3) ν2dh = 28.27 (4) nNh =1.846660 (5) |r2Na|/f = 0.34495 (6A) |r2a|/r1b = 1.01193 (7) Bf/f =0.75410 (9) n2dh = 2.003300

In the lens system according to Example 8, the lens having highestrefractive power at d-line (wavelength=587.6 nm) in the second lensgroup G2 is the lens L24. The lens L21 is the negative lens LNsatisfying conditional expression (4), and the negative lens L2N havinga concave surface facing the object side. Here, the radius of curvatureof the object side surface of the lens L21 is r2Na and r2a, and theradius of curvature of the image side surface of the lens L12R is r1b.

FIGS. 16A and 16B are graphs showing various aberrations of the lenssystem according to Example 8, in which FIG. 16A shows upon focusing oninfinity (β=0.0), and FIG. 16B shows upon focusing on a close object(β=− 1/30).

As is apparent from the respective graphs, the lens system according toExample 8 shows superb optical performance as a result of goodcorrections to various aberrations.

EXAMPLE 9

FIG. 17 is a sectional view showing a lens configuration of a lenssystem according to Example 9.

The lens system according to Example 9 is composed of, in order from anobject side along an optical axis, a first lens group G1 having positiverefractive power, and a second lens group G2 having positive refractivepower. An aperture stop S is disposed between the first lens group G1and the second lens group G2.

The first lens group G1 is composed of, in order from the object sidealong the optical axis, a sub-lens group GS11 having positive refractivepower, and a sub-lens group GS12 having negative refractive power. Thesub-lens group GS11 is constructed by a positive meniscus lens L111having a convex surface facing the object side with stronger refractivepower than an image side surface. The sub-lens group GS12 is constructedby, in order from the object side along the optical axis, a negativemeniscus lens L121 having a convex surface facing the object side and apositive meniscus lens L12R having a concave surface facing the imageside.

The second lens group G2 is composed of, in order from the object sidealong the optical axis, a cemented lens constructed by a double concavenegative lens L21 whose radius of curvature of the object side surfaceis smaller than that of the image side surface cemented with a doubleconvex positive lens L22 whose radius of curvature of the image sidesurface is smaller than that of the object side surface, a positivemeniscus lens L23 having a convex surface facing the image side, and adouble convex positive lens L24 disposed to the most image side withstronger refractive power on the image side surface with respect to theobject side surface. Light ray come out from the lens L24 forms an imageon the image plane I.

Focusing on a close object is carried out by moving the first lens groupG1 and the second lens group G2 toward the object side in a body.

In Example 9, although the lens element L111 is composed of a singlelens, the lens element L111 may be a cemented lens. In this case,spherical aberration and longitudinal chromatic aberration can beexcellently corrected.

Various values associated with the lens system according to Example 9are listed in Table 9.

TABLE 9 [Specifications] f = 51.60 FNO = 2.06 ω = 25.23 Y = 24.00 TL =107.25 [Lens Data] i r d nd νd 1 50.5356 4.0000 1.799516 42.22 2154.2377 1.0000 3 24.8887 2.0000 1.834807 42.71 4 16.7477 7.0000 517.2732 2.5000 2.019600 21.45 6 16.8254 8.0000 7 ∞ 9.0000 Aperture StopS 8 −20.4694 1.5000 1.860740 23.06 9 547.0172 8.2000 1.754999 52.32 10 −25.9174 1.0000 11  −113.9151 4.3000 1.834807 42.71 12  −37.7986 0.500013  391.9984 3.3000 1.903658 31.31 14  −161.9377 (Bf) [VariableDistances] INF CLD R = ∞ 1.66 β = 0.0 −1/30 Bf = 54.9471 56.6672 [Valuesfor Conditional Expressions] (2) f/f1 = 0.00263 (3) ν2dh = 31.31 (4) nNh= 1.834807 (L121) (4) nNh = 1.860740 (L21) (5) |r2Na|/f = 0.39668 (6A)|r2a|/r1b = 1.21658 (7) Bf/f = 1.06482 (9) n2dh = 1.903658

In the lens system according to Example 9, the lens having highestrefractive power at d-line (wavelength=587.6 nm) composing the secondlens group G2 is the lens L24. The lenses L121 and L21 are the negativelens LN satisfying conditional expression (4), and the lens L21 is alsothe negative lens L2N having a concave surface facing the object side.Here, the radius of curvature of the object side surface of the lens L21is r2Na and r2a, and the radius of curvature of the image side surfaceof the lens L12R is r1b.

FIGS. 18A and 18B are graphs showing various aberrations of the lenssystem according to Example 9, in which FIG. 18A shows upon focusing oninfinity (β=0.0), and FIG. 18B shows upon focusing on a close object(β=− 1/30).

As is apparent from the respective graphs, the lens system according toExample 9 shows superb optical performance as a result of goodcorrections to various aberrations.

As described above, the present invention makes it possible to provide alens system having high optical performance, a large aperture ratio.

Then, a camera, which is an optical apparatus equipped with the lenssystem according to the present embodiment, is explained. Although acase that the lens system according to Example 1 is installed isexplained, the same result can be obtained by a lens system according toany other Example.

FIG. 19 is a sectional view showing a camera equipped with the lenssystem according to Example 1.

In FIG. 19, the camera 1 is a single-lens reflex digital camera 1equipped with the lens system according to Example 1 as an imaging lens2. In the camera 1, light emitted from an unillustrated object isconverged by the imaging lens 2, reflected by a quick return mirror 3,and focused on a focusing screen 4. The light focused on the focusingscreen 4 is reflected a plurality of times in a pentagonal roof prism 5,and led to an eyepiece 6. Accordingly, a photographer can observe theobject image as an erected image through the eyepiece 6.

When the photographer presses an unillustrated release button all theway down, the quick return mirror 3 is retracted from the optical path,a focal plane shutter 8 is also retracted from the optical path, and thelight from the unillustrated object is formed an object image on animaging device 7. Accordingly, the light emitted from the object iscaptured by the imaging device 7, and stored in an unillustrated memoryas a photographed image of the object. In this manner, the photographercan take a picture of an object by the camera 1.

With installing the lens system according to Example 1 as an imaginglens 2 into the camera 1, it becomes possible to realize a camera havinghigh optical performance.

Then, an outline of a method for manufacturing a lens system including,in order from an object, a first lens group G1 and a second lens groupG2 according to the present embodiment is explained below with referenceto FIG. 20.

At first, a meniscus lens having a convex surface facing the object sideis disposed in a sub-lens group GS12 having negative refractive power,which is disposed in a lens barrel having a cylindrical shape.

Then, the first lens group G1 and the second lens group G2 are disposedin such a manner that the first lens group G1 is constructed by asub-lens group GS11 having positive refractive power and the sub-lensgroup GS12 having negative refractive power, and the second lens groupG2 has positive refractive power with satisfying the followingconditional expressions (1) and (2):

1.910<ndh   (1)

−0.400<f/f1<0.500   (2)

where ndh denotes a refractive index at d-line (wavelength=587.6 nm) ofa lens having the highest refractive index at d-line composing the lenssystem, f1 denotes a focal length of the first lens group, and f denotesa focal length of the lens system.

Moreover, an outline of a method for manufacturing a lens systemincluding, in order from an object, a first lens group G1 and a secondlens group G2 according to the present embodiment is explained belowwith reference to FIG. 21.

At first, a meniscus lens having a convex surface facing the object sideis disposed in a sub-lens group GS12 having negative refractive power,which is disposed in a lens barrel having a cylindrical shape.

Then, the first lens group G1 and the second lens group G2 are disposedin such a manner that the first lens group G1 is constructed by asub-lens group GS11 having positive refractive power and the sub-lensgroup GS12 having negative refractive power, and the second lens groupG2 has positive refractive power with satisfying the followingconditional expressions (4) and (2):

1.820<nNh   (4)

−0.400<f/f1<0.500   (2)

where nNh denotes a refractive index at d-line (wavelength=587.6 nm) ofa negative lens having a high refractive index at d-line composing thelens system, f1 denotes a focal length of the first lens group, and fdenotes a focal length of the lens system.

Incidentally, the following description may suitably be applied withinlimits that do not deteriorate optical performance.

In the above described explanations and Examples, although lens systemshaving a two-lens-group configuration has been shown, the abovedescribed lens configuration can be applied to other lens configurationssuch as a three-lens-group configuration and a four-lens-groupconfiguration. Specifically, a lens configuration in which a positivelens group or a negative lens group is added to the most object side,and a lens configuration in which a positive lens group or a negativelens group is added to the most image side may be listed. A lensconfiguration that a positive lens group or a negative lens group isadded between the first lens group and the second lens group can bementioned.

In order to vary focusing from infinity to a close object, a portion ofa lens group, a single lens group, or a plurality of lens groups may bemoved along the optical axis as a focusing lens group. In this case, thefocusing lens group can be used for auto focus, and suitable for beingdriven by a motor such as an ultrasonic motor. It is particularlypreferable that at least a portion of the first lens group or the secondlens group is moved as the focusing lens group.

Moreover, a lens group or a portion of a lens group may be moved as avibration reduction lens group in a direction perpendicular to theoptical axis thereby correcting an image blur caused by a camera shake.In particular, at least a portion of the first lens group or the secondlens group is preferably made as a vibration reduction lens group.

Although an aperture stop is preferably disposed between the first lensgroup and the second lens group, the function may be substituted by alens frame without disposing a member as an aperture stop.

Moreover, the lens surface of the lenses configuring the lens system maybe coated with an anti-reflection film having a high transmittance in abroad wave range. With this contrivance, it is feasible to attain thehigh contrast and the high optical performance by reducing a flare andghost.

The present embodiment only shows a specific example for the purpose ofbetter understanding of the present application. Accordingly, it isneedless to say that the present application in its broader aspect isnot limited to the specific details and representative devices.

1-26. (canceled)
 27. A lens system comprising, in order from an objectside: a first lens group; and a second lens group having positiverefractive power; the first lens group including a sub-lens group havingpositive refractive power, and a sub-lens group having negativerefractive power, the sub-lens group having negative refractive powerincluding a meniscus lens having a convex surface facing the objectside, the lens system including at least a first negative lenssatisfying the following conditional expressions:1.820<nNh−0.400<f/f1<0.500 where nNh denotes a refractive index at d-line(wavelength=587.6 nm) of the first negative lens, f1 denotes a focallength of the first lens group, and f denotes a focal length of the lenssystem, the second lens group including a negative lens having a concavesurface facing the object side, the following conditional expressionfurther being satisfied:0.300<|r2Na/f|<0.600 where r2Na denotes a radius of curvature of theobject side surface of the negative lens having a concave surface facingthe object side.
 28. The lens system according to claim 27, wherein thefirst lens group consists of the sub-lens group having positiverefractive power and the sub-lens group having negative refractivepower.
 29. The lens system according to claim 27, wherein an aperturestop is disposed between the first lens group and the second lens group.30. The lens system according to claim 27, wherein the negative lenshaving a concave surface facing the object side is disposed to the mostobject side in the second lens group.
 31. The lens system according toclaim 27, wherein the following conditional expression is satisfied:12.0<νdN<24.0 where νdN denotes an Abbe number at d-line of the firstnegative lens.
 32. The lens system according to claim 27, wherein thefirst negative lens has a meniscus shape.
 33. The lens system accordingto claim 27, wherein the first negative lens is disposed to the mostimage side of the first lens group, and has a concave surface facing theimage side.
 34. An optical apparatus equipped with the lens systemaccording to claim
 27. 35. A method for manufacturing a lens systemcomprising, in order from an object side along an optical axis, a firstlens group and a second lens group, the method including steps of:disposing a meniscus lens having a convex surface facing the object sidein a sub-lens group having negative refractive power; disposing thefirst lens group and the second lens group in such a manner that thefirst lens group consists of a sub-lens group having positive refractivepower and the sub-lens group having negative refractive power, thesecond lens group has positive refractive power, and the followingconditional expressions are satisfied:1.820<nNh−0.400<f/f1<0.500 where nNh denotes a refractive index at d-line(wavelength=587.6 nm) of a negative lens of the lens system, f1 denotesa focal length of the first lens group, and f denotes a focal length ofthe lens system, the second lens being disposed to include a negativelens having a concave surface facing the object side, and the followingconditional expression further being satisfied:0.300<|r2Na/f|<0.600 where r2Na denotes a radius of curvature of theobject side surface of the negative lens having a concave surface facingthe object side.
 36. A lens system comprising, in order from an objectside: a first lens group; and a second lens group having positiverefractive power; the first lens group including a sub-lens group havingpositive refractive power, and a sub-lens group having negativerefractive power, the sub-lens group having negative refractive powerincluding a meniscus lens having a convex surface facing the objectside, the lens system including at least one negative lens satisfyingthe following conditional expressions:1.820<nNh−0.400<f/f1<0.5000.800<|r2a|/r1b<1.200 where nNh denotes a refractive index at d-line(wavelength=587.6 nm) of the negative lens, f1 denotes a focal length ofthe first lens group, f denotes a focal length of the lens system, r1bdenotes a radius of curvature of the most image side lens surface in thefirst lens group, and r2a denotes a radius of curvature of the mostobject side lens surface in the second lens group.
 37. The lens systemaccording to claim 36, wherein the first lens group consists of thesub-lens group having positive refractive power and the sub-lens grouphaving negative refractive power.
 38. The lens system according to claim36, wherein an aperture stop is disposed between the first lens groupand the second lens group.
 39. The lens system according to claim 36,wherein the second lens group includes at least one negative lens havinga concave surface facing the object side, and the following conditionalexpression is satisfied:0.300<|r2Na|/f<0.600 where r2Na denotes a radius of curvature of theobject side surface of the at least one negative lens having a concavesurface facing the object side.
 40. The lens system according to claim36, wherein the at least one negative lens having a concave surfacefacing the object side is disposed to the most object side in the secondlens group.
 41. The lens system according to claim 36, wherein thefollowing conditional expression is satisfied:12.0<νdN<24.0 where νdN denotes an Abbe number at d-line of the negativelens.
 42. The lens system according to claim 36, wherein the negativelens has a meniscus shape.
 43. The lens system according to claim 36,wherein the negative lens is disposed to the most image side of thefirst lens group, and has a concave surface facing the image side. 44.An optical apparatus equipped with the lens system according to claim36.
 45. A method for manufacturing a lens system comprising, in orderfrom an object side along an optical axis, a first lens group and asecond lens group including steps of: disposing a meniscus lens having aconvex surface facing the object side in a sub-lens group havingnegative refractive power; disposing the first lens group and the secondlens group in such a manner that the first lens group consists of asub-lens group having positive refractive power and the sub-lens grouphaving negative refractive power, the second lens group has positiverefractive power, and the following conditional expressions aresatisfied:1.820<nNh−0.400<f/f1<0.5000.800<|r2a|/r1b<1.200 where nNh denotes a refractive index at d-line(wavelength=587.6 nm) of a negative lens of the lens system, f1 denotesa focal length of the first lens group, f denotes a focal length of thelens system, rib denotes a radius of curvature of the most image sidelens surface in the first lens group, and r2a denotes a radius ofcurvature of the most object side lens surface in the second lens group.